Temperature-responsive electrical switch

ABSTRACT

An improved, temperature-responsive electrical switch is provided which is actuated at temperatures above 250* C., the switch utilizing fusible pellets prepared from imide or substituted melamine compounds.

Umted'States Patent 1191 1111 3,727,164 Cartier et al. Apr. 10, 1973 54 TEMPERATURE.RESPONSIVE 3,518,961 7/1970 Kovac ..l16/11.4.5

ELECTRICAL SWITCH 3,054,378 9/1962 Bienfait ..116/114.5

[75] Inventors: Michael D. Cartier; Gaylord L.

[73] Assignee: Minnesota Mining and Manufacturmg Company, St Paul, Minn. Attorney Kinney, Alexander, Sell, Steldt & Delahunt [22] Filed: July 14, 1972 211 Appl. No.: 271,745 ABSTRACT An improved, temperature-responsive electrical 52 us. c1 ..337/405, 337/409 switch is Provided which is actuated at temperatures [51] Int. Cl. ..HOlh 37/76 abo e 25 C-, the Switch utilizing fusible pellets [58] Field of Search ..337/407, 405, 406, prepared from imide or substituted melamine com- 337/408, 409, 401; 116/1145 pounds.

[56] References Cited UNITED STATES PATENTS 7 Claim, 4 Drawing Figures 3,519,972 7/1970 Merrill ..337/407 Groff, both of St. Paul, Minn.

Primary Examiner-Harold Broome PATENTEBAER 1 01975 FIG. 1

III

electrical current therethrough until a predetermined temperature is reached.

Typical of such temperature-responsive switches are those described, e.g., in Merrill, U.S. Pat. No. 3,519,972; Merrill, U.S. Pat. No. 3,505,630; Merrill, U.S. Pat. No. 3,291,945; Merrill, U.S. Pat. No.

3,180,958; Massar, U.S. Pat. No. 2,934,628; Weese,

U.S. Pat. No. 3,313,312; Bahr, U.S. Pat. No. 2,247,902; Ebensteiner, U.S. Pat. No. 3,281,559; Mil ton, U.S. Pat. No. 2,955,179; Denton, U.S. Pat. No. 3,155,800; Delaney, U.S. Pat. No. 1,368,230; Spracher, U.S. Pat. No. 2,442,830; Derby et al., U.S. Pat. No. 2,516,964; Pierce, U.S. Pat. No. 968,406; Thomas, U.S. Pat. No. 2,996,591; Chabala, U.S. Pat. No. 3,1 18,992; and Fister, U.S. Pat. No. 3,281,557.

Each type of temperature-responsive switch typically contains means movable between a first position and a second position to either open or close an electrical path within the switch. Such means is ordinarily actuated by the collapse of a normally solid, fusible pellet, as described in the aforementioned patents.

Fusible pellets for use in these temperature-responsive electrical switches have been made from a variety of materials. For example, they have been made from metallic alloys e.g., bismuth, lead, tin or antimony al loys), waxes, powdered ceramic material mixed with fusible binder, etc.

In spite of the'wide variety of temperature-responsive switches available, and in spite of the existence of a wide variety of fusible pellets, there has not heretofore been provided a suitable temperature-responsive electrical switch which will operate reliably above temperatures on the order of 250 C. or more. The present invention provides such a temperature-responsive electrical switch.

The invention will be described in more detail hereinafter with reference to the drawings wherein like reference characters refer to the same part throughout the several views and in which:

FIG. 1 is an elevation of one completed switch of this invention;

FIG. 2 is a longitudinal, cross-sectional view of a typical temperature-responsive electrical switch of the invention;

FIG. 3 is a cross-sectional view of the portion of the casing of the switch shown in FIG. 2; and

FIG. 4 is a cross-sectional view of a portion of one of the leads of the switch of FIG. 2.

FIG. 1 shows a temperature-responsive electrical switch made in accordance with the invention and having conductive leads 12 and 14 extending into the casing of the switch which is encapsulated in electrically insulative resin 16.

In FIG. 2 there is shown a longitudinal cross-sectional view of a temperature-responsive electrical switch 10 of the general type described in U.S. Pat. No. 3,519,972, the switch comprising a cylindrical, tubular, electrically and thermally conductive casing 18 having closures 20 and 22 at the ends thereof. Closure 20 is an electrically nonconductive closure.

A first conductor 12 conductively contacts the casing 18 at the integral closure 22. A second conductor 14 extends into the casing 18 through the electrically nonconductive closure 20. A normally solid fusible pellet 24 is spring-biased to exert outward pressure against one of the closures, such as the closure 22.

An electrically conductive member 26 is slidably mounted within the casing 18 and has a slidable, resilient, peripheral conductive engagement with the conductive casing 18, such as by outwardly resilient integral teeth which outwardly press against the inner surface of the casing 18. Member 26 also has a conductive central portion in electrical contact with the second conductor 14 via conductive disc 38 in one position of member 26 and out of electrical contact with said second conductor 14 in another position of said member 26.

A compression spring construction inside casing 18 serves as means to change the electric flow condition between the casing and conductor 14 upon collapse of the pellet 24, such as by leftward movement of conductive member 26 away from electrical contact with conductor l4.

The nonconductive closure 20 may have a central opening 28 surrounding the second conductor 14, and the closure 20 may be secured in the conductive casing 18 as shown in the drawing.

The compression spring construction may include a relativelystrong compression spring 30 between the pellet 24 and the electrically conductive member 26. Such compression spring construction also may have a relatively weak compression spring 32 between the closure 20 and the electrically conductive member 26. As long as the pellet 24 does not collapse, the stronger spring 30 holds the slidable, conductive member 26 against disc 38 and head 34 of second conductor 14 so that electrical current may pass through first conductor 12, casing 18, conductive member 26, disc 38 and then second conductor 14. However, when the pellet 24 reaches a temperature above 250 C. (depending on the particular type of compound used in making the pellet), the material of pellet 24 becomes fluid almost instantaneously and flows around disc 36 and thus allows the stronger spring 30 to expand towards closure 14 so that its spring load becomes less than the spring load of the weaker spring 32. This permits spring 32 to move the conductive member 26 and the disc 38 away from head 34. Consequently, the conductive member 26 is moved out of electrical contact with head 34 of second conductor Maud thus breaks the electrical flow condition between the first conductor 12 and the second conductor 14 to stop the flow of current through switch 10.

In order to make the conventional copper or brass casing normally used in electrical switches more temperature-resistant, they must be overcoated with thin layers of various other metals. As shown in FIG. 3, the casing 18 can be overcoated with a thin layer 40, e.g.,

nickel, followed by a thin overcoating 42 of another metal such as silver or gold. These thin overcoatings may vary from about 2.5 microns to about 50 microns in thickness, and they have been shown to reduce or retard oxidation of the conventional copper or brass casings at temperatures above 250 C. 1

In orderto prevent the sticking or welding at high temperatures and pressures of disc 38 and second conductor 14, these parts are coated or treated with overlayers of metal of a type which isharder than silver. Thus, in FIG. 4 there is a cross-sectional view of second conductor 14 having a first thin overcoating 44 and a second thin overcoating 46. [t has been found that thin overcoatings of first nickel and then rhodium are quite suitable for use in the switches of this invention. It has also been found that platinum or palladium maybe used in place of rhodium as the second thin overcoating. Typical thickness for the nickel overcoatings ranges from about 2.5 microns to about 7.5 microns. Typical thickness for the rhodium, platinum or palladium overcoatings ranges from about 0.25v microns to about 1.5 microns, although thicker overcoatings can also be used.

The thin nickel overcoatings are normally applied using well-known techniques of electroless plating or electroplating using commercially available plating solutionsQThe rhodium, platinum, or palladium overcoatings are normally applied using well-known electroplating techniques and commercially available plating solutions. i

The encapsulating resin 16 which surrounds the elec-' trical switch is thermally resistant and it retards the entry of air and moisture into the temperature-responsive electrical switch and also assists in anchoring the first and second conductors l2 and 14. Resin 16 is preferably tough and exhibits a high heat-distortion temperature as well as permitting rapid heat transfer therethrough. Preferably, resin l6 is a silicone molding compound which may be filled with glass fibers or fused silica or both. Typical examples of suchresins are those commercially available from Dow Coming in their 300 series which are filled with both short glass fibers and fused silica. Another typical silicone resin is General Electric M C710, a thermosetting silicone resin filled with silica glass fibers (typical cure being 424 hours at 200 C.). Those polyimide and phenolic resins which are quite thermally resistant and tough can also be used.

Resin 16 is normally formed around the switch by either compression or transfer molding. Typical conditions for transfer molding, e.g., the Dow Corning 306 resin are a temperature of 150l 80 C., a dwell of 1-5 minutes, and a molding pressure of about 2-5-30 kilograms per square centimeter, followed by a post-cure of 2 hours at 200 C.

- therethrough. When using, e.g., the Dow Corning 306"resin, the minimum resin thickness is about 0.3 millimeters but it may be as thick as 0.8 millimeters or more. 1

Fusible pellets which are useful in the temperatureresponsive electrical switches of the present invention are those which are'stable (i.e.,, do not melt, significantly sublime or degrade) at temperatures up to at 5 least 250 C. over extended periods of time. The pellets are made from organic compounds having no reactive groups (e.g., acid, hydroxy, acid chloride, halogen,

etc.) and having relatively low vapor pressures.

Although several classes of compounds are known 10 which have melting points above 250 C., there havev only been found two classes of compounds which meet the requirements for use in fusible pellets for high temperature-responsive electrical switches. These classes are the imide compounds and substituted melamine compounds. Exemplary of the substituted melamine compounds is hexaphenyl melamine which rnelts at 304 C. and which has the following structure:

above 250 C. are also useful. Melamine compounds having aromatic substituents are preferred over those having aliphatic substituents. Substituted melamine compounds can be prepared in accordance with the 40 methods described in The Relative Thermal Stability Bentz, .I. C. Petropoulos, J. Applied Polymer Science, 6 (19), 47 (1962).

Exemplary of useful high-melting imide compounds are the following:

45 Diphthalimidodiphenylsulfone, having the structure 5 and having a melting point of 3 24325 C.;

diphthalimidodiphenyl ether, having the structure and having a melting point of 289 C.; and imide of benzophenone dianhydride and o-toluidine, having the structure of Polymer Model Compounds," D. Sheenan, A. P.

and having a melting point of 279 C.

Methods for preparing imides are well known, e.g., as described in US. Pat. Nos. 3,179,630 and 3,179,632 and in various other references.

Fusible pellets are made from the above-described compounds by compressing the powdered compound in a cylindrical cavity. The force applied and the amount of material in the cavity is adjusted in such a manner that about 30-35 fusible pellets are obtained per gram of powdered compound. The fusible pellets (typically having a diameter of 3.2 mm.) should able to sustain an axial load of at least about 4.5 kilograms (measured with a Stokes Tablet Hardness Tester).- For some compounds (for example, hexaphenylmelamine), it is difficult or impossible to obtain a pellet having sufficient strength for use in the temperature-responsive switch. Consequently, a binder material is ordinarily added to such compounds to increase the internal strength of pellets made therefrom. A binder level of about 3-5 percent by weight has been found sufficient for this purpose. Suitable binders which have been used include Scotchcast 265, a thermosetting epoxy resin powder commercially available from the 3M Company; Dow Corning 901 Varnish, a thermosetting silicone resin dissolved in a solvent; and General Electric SR-350, a powdered thermosetting silicone resin. The powdered binder resins are dispersed in the finely divided compound used for the pellet by any method of dry blending. The compound with added binder is then compressed and heated to melt and cure the binder. For example, pellets made using an epoxy resin as a binder may be heated for about 40 minutes at 180 C.

When using silicone resin as the binder material, it is .dissolved in a suitable solvent along with the catalyst therefor, after which the finely divided pellet material is added thereto. The resulting dispersion is heated to drive off the solvent and the resulting solid is ground to a powder, pressed into pellets, and then cured with heat (e.g., 2 hours at 200 C.).

The following nonlimiting examples serve to further illustrate the present invention:

EXAMPLE 1 Hexaphenylmelamine is prepared using the following ingredients:

Cyanun'c chloride 92 grams Diphenyl amine 169 grams These materials are charged to a flask fitted with an acid scrubbing tower and nitrogen gas atmosphere. The flask contents are then heated slowly to 200 C. and held until no more hydrogen chloride gas evolution is observed. The solid product is washed in chloroform and then purified by sublimation at a pressure of 50-1'00 microns Hg and a temperature just below the melting point of the product.

EXAMPLE 2 Diphthalimidodiphenyl sulfone is prepared using the following ingredients:

Diaminodiphenylsulfone 24.8 grams Phthalic anhydride 29.6 grams precipitated imide product is recovered, filtered, and dried.

EXAMPLE 3 A fusible pellet is prepared by first dispersing 0.97 gram of hexaphenylmelamine in 10 ml. of toluene containing 0.06 gram of Dow Corning 901 silicone varnish (55 percent solids) and 0.000216 gram lead octoate (24 percent lead by weight). The resulting dispersion is dried for 1 hour at F.

Pellets were formed from the dry residue using 1,000 pounds axial pressure. Approximately 0.03 grams of material were used for each pellet. The pellets were cured for 30 minutes at C. after which they could withstand an axial force of 10.5 kilograms.

What is claimed is:

1. In a temperature-responsive electrical switch of the type comprising:

a. an electrically and thermally conductive casing having a closure at each end thereof, one of said closures being electrically nonconductive;

b. a first conductor in electrical contact with said casing;

c. a second conductor extending into said casing through said electrically nonconductive closure;

d. a normally solid fusible pellet contained in said casing; and

means contained in said casing for permitting electrical flow through said switch via said first conductor, said casing and said second conductor when said means is in one position and not permitting electrical flow through said switch when said means is in another position; said means being actuated by the collapse of said fusible pellet;

the improvement which comprises a temperature-resistant casing, a normally solid, fusible pellet comprising an imide compound or a substituted melamine compound having a melting temperature of at least 250 C., and a second conductor whose surface bears a first thin overcoating of nickel and a second thin overcoating of rhodium, platinum or palladium, said switch being encapsulated in a thermally resistant resin.

2. The improvement of claim 1 wherein said temperature-resistant casing comprises a copper or brass shell bearing a first thin overcoating of nickel and a second thin overcoating of silver or gold.

diphthalimidodiphenvl ether, and the imide prepared from benzophenon'edianhydride and o-toluidine.

6. The improvement of claim 1 whereinsaidre'sin comprises a cured silicone resin.

7. The improvement of claim 1 wherein said surface of said second conductor bears a first thin'overcoating of nickel and a second thin over-coating of rhodium. 

2. The improvement of claim 1 wherein said temperature-resistant casing comprises a copper or brass shell bearing a first thin overcoating of nickel and a second thin overcoating of silver or gold.
 3. The improvement of claim 2 wherein said casing comprises a brass shell bearing a first thin overcoating of nickel and a second thin overcoating of silver.
 4. The improvement of claim 1 wherein said fusible pellet comprises hexaphenylmelamine.
 5. The improvement of claim 1 wherein said fusible pellet is prepared from a compound selected from the group consisting of diphthalimidodiphenylsulfone, diphthalimidodiphenyl ether, and the imide prepared from benzophenonedianhydride and o-toluidine.
 6. The improvement of claim 1 wherein said resin comprises a cured silicone resin.
 7. The improvement of claim 1 wherein said surface of said second conductor bears a first thin overcoating of nickel and a second thin overcoating of rhodium. 